Such a modulator is used in particular in satellite radio broadcasting systems in which the broadcast signal consists of a time multiplex of a carrier frequency modulated by the picture signal during the active period of one television line and a carrier phase modulated by a digital signal during the line suppression interval. This satellite radio broadcasting system is known in the European Radio Broadcasting Unit (ERU) under the name of "C system". More particularly in this sytem, the digital modulation used is MDP2-4 which consists of a binary modulation which has four states that differ by 90.degree., in which the binary 1 symbol is transmitted by a jump in phase of +90.degree. and the symbol 0 is transmitted by a jump in phase of -90.degree.. The MDP2-4 modulation is completely described in European patent No. EP-A-0032325. It has the salient feature of having an almost constant envelope after filtering. In the C system, the instantaneous digital bit rate is of 20.25 Mbit/s and the duration of a burst is 10 microseconds.
Of course, in the C system receiving equipment, it is necessary to provide a modulated carrier amplification stage, then a division in two channels one of which comprises an analog frequency demodulator which yields the picture signal and the other a digital differential demodulator which yields the data signal at 20.25 Mbit/s.